The goal of the research proposed here is to test the hypothesis that the association between systemic lupus erythematosus and the A-null B1 phenotype of the fourth human complement protein (C4) is not fortuitous but rather that it results from the relatively low covalent binding efficiency of the C4B1 allotypic form of the protein to immune complexes composed of nucleic acids and antibodies directed against them and that the poor binding efficiency of the C4B1 allotype in serum could account, in part, for the inefficient processing and clearance of the immune complexes via C3b receptors of SLE patients of the C4 A-null B1 phenotype. The experiments proposed here focus specifically on the chemistry of the covalent binding properties of the A3 and B1 allotypes of C4 obtained from normal, healthy donors and from donors with SLE. In order to study the covalent binding properties of these forms of C4 it is necessary to carry out experiments under simple, chemically defined conditions. The fluid-phase binding system developed in this laboratory and a newly devised solid-phase assay designed to take advantage of the binding of nucleic acid-protein complexes to nitrocellulose filters will be used in the research proposed here. Both the fluid- and the solid-phase methods permit the determination of the parameters that govern the covalent binding reaction as well as the determination of the chemical nature of the bonds that are formed. Interactions between C4 and DNA and snRNP (ribonucleoprotein complexes of nuclear origin) and immune complexes composed of DNA and snRNP and antibodies will be investigated both in the completely defined fluid- and solid-phase systems and in normal and patient sera. A modified form of the fluid-phase binding system will permit the study of the binding properties of C4 of known allotypes in serum of both normal donors and those with SLE. These experiments with normal and patient sera will be the first time that immune complexes relevant to SLE will have been studied from the point of view of their interaction with the complement system. The experiments proposed here are a direct test of the hypothesis that the reactivity of C4 plays a role in the pathogenesis of SLE. From their results it will be possible to conclude whether or not the known differences in the chemical reactivity of C4A3 and C4B1 are expressed in their interactions with immune complexes of the sort that are found among SLE patients.